Which of the following oxidation numbers is not correctly matched?

P

N a H_{2} P O_{4} = + 5

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N i

[N i (C N)_{6}]^{4 -} = + 2

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P

M g_{2} P_{2} O_{7} = + 6

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C r

(N H_{4})_{2} C r_{2} O_{7} = + 6

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Solution

The correct option is A $P$ in $M g_{2} P_{2} O_{7} = + 6$
Let the oxidation number of a desired atom be $x$ .
Oxidation number of $H = N a = + 1$ , $C N = - 1$ , $O = - 2$ , $M g = + 2$ , $N H_{4} = + 1$
(A) P in $N a H_{2} P O_{4}$ : $1 + 2 \times 1 + x + 4 \times (- 2) = 0$
$x - 5 = 0$ or $x = + 5$
(B)Ni in $[N i (C N)_{6}]^{4 -}$ : $x + 6 \times (- 1) = - 4$
$x - 6 = - 4$ or $x = + 2$
(C) P in $M g_{2} P_{2} O_{7}$ : $2 \times 2 + 2 \times x + 7 \times (- 2) = 0$
$2 x - 10 = 0$ or $x = + 5$
(D) Cr in $(N H_{4})_{2} C r_{2} O_{7}$ : $2 \times 1 + 2 \times x + 7 \times (- 2) = 0$
$2 x - 12 = 0$ or $x = + 6$

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